Cutting blades are ground and sharpened at angles that are optimized for particular purposes. When the blade becomes dulled by use, the user may resharpen the blade manually using a sharpening stone. The blade is rubbed on the stone to restore the original angle and edge configuration. This requires great skill in holding the blade at the correct angle during the process. Various devices have been provided to ensure that the correct angle is provided. They generally clamp onto the back of the blade and provide a planar surface to lay against the grinding surface at the correct angle. They are exemplified by the following U.S. Patents:
U.S. Pat. No. 880,765 issued Mar. 3, 1908 to Utter PA0 U.S. Pat. No. 2,782,570 issued Feb. 26, 1957 to Ische PA0 U.S. Pat. No. 1,103,427 issued Jul. 14, 1914 to Neider
These are generally sturdily constructed so that they clamp firmly in position and resist dislodging by the grinding action. Unfortunately, the surface that lies against the grinding surface becomes ground down with use, so that the angle changes. Another problem presented by the grinding guides of the prior art is that they are generally not useful for the curved tip of certain blades, and they are of limited length, so that they are not useful for long blades, such as a machette. Furthermore, as the guide is ground, the material of the guide is deposited on the grinding surface, which may load the stone so that it loses its utility. This is especially true of aluminum guides which are in common use.